Infrared-based imaging and detection devices have found widespread use and are known to those skilled in the art as “FLIRs” (an acronym derived from “Forward-Looking Infrared”). Since infrared-based imaging devices sense temperature differences between objects in a field of view, they are particularly useful at night and in daytime during periods of reduced visibility. In contrast to the visible spectrum, the infrared spectrum frequently provides high contrast images at night and during periods of reduced visibility. This results from a number of factors, a first factor being that infrared-based imaging devices sense temperature differences between objects in a field of view. At night and during daytime periods of reduced visibility there is frequently a significant thermal contrast between objects of interest (such as, for example, vehicles and persons) and a background. A second factor results from the fact that during periods of reduced visibility atmospheric obscurants (such as, for example, smoke or sand) may attenuate visible light to a far greater degree than infrared radiation.
Infrared-based imaging devices use infrared detectors to detect infrared radiation emanating from objects in a field of view. Conventional infrared detectors are constructed from exotic semiconductor materials such as HgCdTe and InSb. A particular limitation of conventional infrared detectors is their need to be cooled to cryogenic temperatures in order to achieve desired levels of thermal sensitivity. The requirement for active cooling increases the cost, complexity and power consumption of such infrared imaging devices, and distinguishes them from visible-spectrum video cameras that do not require such active cooling. In addition, the need for cooling apparatus introduces complexities into the design of optical elements used in combination with the infrared detectors of infrared-based imaging devices.
Infrared-based imaging devices using detectors that are not cryogenically cooled are known, but lack fast response times for operation in dynamic situations. Uncooled sensors also lack narrow-bandwidth spectral responses for surveillance and identification.
For purposes of surveillance and identification, it is often useful to limit the response of a sensor to a particular bandwidth. If an object that is to be detected is known to have a unique thermal signature because it emanates infrared radiation of a particular wavelength or combination of wavelengths, tuning the response of the sensor to the particular wavelength or combination of wavelengths serves a filtering function by eliminating other objects that have different thermal signatures from consideration.
Progress has been made in providing tunability for infrared detectors through the introduction of antenna-coupled infrared detectors. In antenna-coupled infrared detectors, an antenna element sensitive to infrared radiation is coupled to a conventional infrared photodetector. Such antenna-coupled infrared photodetectors are frequency-tunable and may have an adjustable polarization response. Nonetheless, infrared-based imaging or detection devices that are to be used in dynamic situations require the use of cooled detectors, thereby increasing the cost and complexity of such devices.
Accordingly, those skilled in the art desire improved designs for infrared sensor elements and sensors that eliminate the need for cooling; achieve or exceed the sensitivity of conventional infrared detectors; and preserve the advantages of antenna-coupled detectors.